Many circuit designs include both digital and analog circuits. For example, a digitally controlled radio contains both an analog circuit (comprised of radio frequency (RF) and intermediate frequency (IF) circuits), and a digital circuit for controlling the operation of the analog RF/IF portions of the radio. Mixed circuit designs are highly susceptible to both cross-talk between the analog and digital circuits and RF/IF signal degradation in the analog circuit due to digital noise. Accordingly, such mixed circuit designs have been implemented on multiple circuit boards shielded from each other to minimize the occurrence of radio frequency interference (RFI) and electromagnetic interference (EMI). However, the installation of the separate circuit boards for the analog and digital circuits in different housings interconnected with shielded and/or filtered cables provides an unsatisfactory solution to RFI/EMI interference concerns in view of the increased costs associated with multi-board fabrication, interconnection and housing.
With the advent of integrated circuit and microcircuit technologies, it has become increasingly possible and important to fabricate mixed circuit designs on a single circuit board or card. Shielding of the circuits continues to be a significant concern. In one shielding method, the analog and digital circuits are fabricated on a single card with an isolation zone placed therebetween to separate the mixed circuits from each other and provide some RFI/EMI isolation. While use of an isolation zone obviates the need to manufacture and interconnect multiple circuit boards, this isolation method is unsatisfactory because an isolation zone inefficiently utilizes the limited available space on the circuit board. In another shielding approach, the analog and digital circuits are fabricated on different circuit boards placed within the separated and isolated sections of an "egg carton" circuit card holder. This method successfully provides a single, space efficient "circuit card" for the mixed circuit design, but from a fabrication standpoint is undesirable due to the added costs associated with fabricating the egg carton and the multiple circuit boards that are inserted therein and must be interconnected for circuit operation.
In view of the foregoing, there is a need for an effective and efficient isolation system that will enable both digital and analog circuit areas to be placed adjacent to each other on a single circuit board without cross-talk or RF/IF signal degradation. Such an isolation system should also enable circuit areas operating at different frequencies to be adjacently fabricated.